Population density influenced the stimulatory or inhibitory effect of inoculated Bacillus sp. on mustard (Brassica juncea L.) under water deficit stress

Abstract

Rhizobacteria stimulate plant growth, however, for maximizing their benefits, plants need to be inoculated with the optimal inoculum density that determines stimulatory or inhibitory effects of the inoculated rhizobacteria. In the present study under water deficit stress, influence of different inoculum densities of Bacillus sp. strain MRD-17 and Bacillus casamancensis strain MKS-6 on seed germination in-vitro and growth of mustard in pot experiment were investigated. Rhizobacterial population densities ranging from 105 to 107 cfu/ mL, were found to stimulate seedling fresh weight in-vitro; and shoot and root fresh weight at 30 days after sowing (DAS) in pot experiment, under control and water deficit stress condition. However, higher inoculum densities (1011 cfu/ mL) of both rhizobacteria had an inhibitory effect on seedling vigour and plant growth. Inoculation of seeds with stimulatory and inhibitory population in a pot experiment, under sterile condition had beneficial and adverse effect on plant growth, respectively. However, under unsterile conditions, the beneficial effects of stimulatory population were lowered while adverse effects of inhibitory population could only be observed on root fresh weight. Population of both rhizobacteria on rhizoplane and endorhizosphere were counted. Under sterile condition, rhizoplane population of inoculated bacteria of more than 109 cfu/ g was inhibitory to the plant growth; while rhizobacterial population of 106 to 108 cfu/ g on rhizoplane improved plant growth. Lower population densities of the inoculated rhizobacteria were observed on rhizoplane, under unsterile condition. Thus, population densities of inoculated rhizobacteria on roots had major impacts on seedling vigour and plant growth.